How do I change the default settings of export operators?
You don't necessarily need to write a custom operator to re-use the existing one. You can call the operator in your layout and change its properties, see How to pass multiple operator properties via UI layout?
import bpy


class HelloWorldPanel(bpy.types.Panel):
    &quot;&quot;&quot;Creates a Panel in the Object properties window&quot;&quot;&quot;
    bl_label = &quot;Hello World Panel&quot;
    bl_idname = &quot;OBJECT_PT_hello&quot;
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = &quot;object&quot;

    def draw(self, context):
        layout = self.layout
        op = layout.operator(&quot;export_mesh.stl&quot;, text=&quot;My Custom STL Exporter&quot;)
        op.use_selection = True
        op.batch_mode = 'OBJECT'


def register():
    bpy.utils.register_class(HelloWorldPanel)


def unregister():
    bpy.utils.unregister_class(HelloWorldPanel)


if __name__ == &quot;__main__&quot;:
    register()


You can find the operator's parameters in your blender install path / 2.** / Scripts / addons / io_mesh_stl / __init__.py under the ExportSTL class.  :
use_selection: BoolProperty(
    name=&quot;Selection Only&quot;,
    description=&quot;Export selected objects only&quot;,
    default=False,
)
global_scale: FloatProperty(
    name=&quot;Scale&quot;,
    min=0.01, max=1000.0,
    default=1.0,
)
use_scene_unit: BoolProperty(
    name=&quot;Scene Unit&quot;,
    description=&quot;Apply current scene's unit (as defined by unit scale) to exported data&quot;,
    default=False,
)
ascii: BoolProperty(
    name=&quot;Ascii&quot;,
    description=&quot;Save the file in ASCII file format&quot;,
    default=False,
)
use_mesh_modifiers: BoolProperty(
    name=&quot;Apply Modifiers&quot;,
    description=&quot;Apply the modifiers before saving&quot;,
    default=True,
)
batch_mode: EnumProperty(
    name=&quot;Batch Mode&quot;,
    items=(
        ('OFF', &quot;Off&quot;, &quot;All data in one file&quot;),
        ('OBJECT', &quot;Object&quot;, &quot;Each object as a file&quot;),
    ),
)